1. Field of the Invention
The present invention relates to a solid-state imaging device and driving a solid-state imaging device, for example, a uncooled-type solid-state imaging element detecting a light having a long wave length.
2. Related Art
An image sensor detecting a light ray in bands of wavelengths of 8 to 12 micrometers (μm) including an infrared band, a terahertz band, a millimeter-wave band, and the like has high sensitivity to a light ray emitted from an object having a temperature near room temperature. Accordingly, such an image sensor has been applied to a security camera, an on-vehicle forward-looking camera or the like. In recent years, following development of MEMS (Micro-Electro-Mechanical System) process, a thermal-type optical sensor (a uncooled-type optical sensor) sensing a light ray without cooling an element has become popular as an image sensor.
The uncooled-type optical sensor irradiates a light ray condensed by, for example, a far-infrared lens (Ge lens) onto each of pixels and converts heat generated by the light ray into an electric signal. The uncooled-type optical sensor generates an image based on the generated electric signal. The uncooled-type optical sensor of this type needs to detect the light ray in a state in which the pixels are thermally separated from a semiconductor substrate. Because each of the pixels is continuously heated by the light ray, it is difficult for the conventional uncooled-type optical sensor to detect an electric signal in a dark state (a state in which light is not incident on each pixel). A signal voltage obtained by the thermoelectric conversion is low as compared with a voltage of the electric signal (i.e., a bias component) in the dark state. However, in the uncooled-type sensor, bias components cannot be eliminated from the respective pixels for the above-stated reason. Due to this, at the time of starting the sensor with the lens covered with a lens cover, the signal in the dark state (a fixed pattern) output from each pixel is stored in a frame memory in advance. Thereafter, the fixed pattern is subtracted from the detected electric signal corresponding to each pixel, thereby outputting signal components corresponding to an incident light.
In this case, however, it is necessary to additionally provide a circuit for subtracting the fixed pattern from each electric signal, disadvantageously resulting in an increase in a size of the sensor. Furthermore, if the fixed pattern has temporal change, then an initial fixed pattern cannot be used and it is disadvantageously necessary to regularly update the fixed pattern.
JP-A 2004-085331 (KOKAI) discloses a thermal-type infrared image pickup device provided with including a mechanical switch so as to be able to quickly respond to an infrared ray. This mechanical switch causes rapid thermal conduction between a cell part and a semiconductor substrate in a period in which no electric signals are detected from respective pixels, and thermally separates the cell part from the semiconductor substrate in a signal detection period. The thermal-type infrared imaging device disclosed therein can, therefore, pick up an image at high speed. However, the thermal-type infrared imaging device disclosed therein is unable to read signals output from the respective pixels in a state in which the cell part is shorted to the semiconductor substrate, that is, in a thermally reset state. As a result, 1/f noise and irregularities in fixed pattern among the pixels cannot be eliminated.